Modern communications networks comprise a plurality of endpoints (such as telephones and computers) linked together via a corresponding plurality of lines, allowing the endpoints to be interconnected to route voice, video and data traffic (hereinafter collectively referred to as "data" or "payload") from any one endpoint through the network to any other endpoint. Interconnection of the lines is conventionally performed at central locations (or "offices") that contain one or more switches.
Conventional analog switches comprise a matrix of interconnects that allow the switch to couple any two of the lines together, thereby creating a connection and allowing data to cross from one line to the other. Although most lines in an analog network carry only a single stream of analog data, time division multiplexing ("TDM") has traditionally been employed to divide a single physical line into fixed-length time slices, allowing multiple analog data streams to be carried in the single physical line. While TDM increases the overall utilization of a given line, the length of each time-slice is immutable. Thus, a given data stream is allocated neither more nor less than its fixed time-slice.
More modern telecommunications networks operate on digital (including digitized analog) data. Persons using the network can readily perceive the resulting increases in data fidelity and transmission rate. However, digital data have other advantages that are not so apparent to the user.
One significant such advantage is that a stream of digital data can readily be divided into segments. The segments can be of varying length or fixed length and may be alternatively called "blocks," "packets," "frames" or "cells." Header information, including the path through the network that the segment is to take or the destination to which the segment is heading, is typically provided with the segments. Depending upon the specific protocol, each segment may also be provided with error checking information. Therefore, each segment can be switched individually through the network. In fact, segments corresponding to multiple data streams can be carried on a single line, but the overall bandwidth of the line can be allocated flexibly, such that each stream is allocated bandwidth as needed (enabling so-called "Bandwidth-ON-Demand"). Thus, switching of digital data is not required to suffer the fixed time-slice disadvantages of TDM.
Asynchronous Transfer Mode ("ATM") is becoming an important standard for communication of digital data. In ATM parlance, the segments are contained in "cells." Cells are fixed at 53 octets in length. The leading 5 octets contain header information, while the remaining 48 octets contain the segment itself. The fixed length of the cells allow them to be transported quickly through a network.
It has been found that general purpose computers can be made to control switches. In such computer-controlled switches, a processor provides central control for the communication of data among ports (coupled to inbound and outbound lines) coupled to the processor by a bus. Segments of data are communicated from one port to the processor and back out to another port, the processor acting as bus master at all times. Some computer-controlled switches employ TDM to divide the processor/bus bandwidth evenly among the various ports. Whether or not TDM is employed, the centralized nature of this communication places heavy burdens on the processor and the bus, limiting the effective bandwidth of the computer-controlled switch.
Often, such computer-controlled switches have an open, modular architecture, allowing the switches to be easily expanded. Accordingly, the ports are provided on port cards. However, the port cards have always been under direct control of ("slaved to") the processor.
Accordingly, what is needed in the art is a computer-controlled cell switch that can switch cells reliably and quickly by having decentralized control of cell communication. Preferably, the cell switch should be modular and amenable to an accepted bus standard. More preferably, the cell switch should be amenable to ATM cells.